JP2000017376A - Non-heat treated steel for hot forging - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel for hot forging, having sufficient tensile strength and yield strength and high toughness even in the part practically free from working in particular, while obviating the necessity of aging treatment as well as control of cooling velocity after hot forging. SOLUTION: This steel has a composition consisting of, by mass, >0.05-<0.15% C, 0.005-2.0% Si, >2.0-5.0% Mn, 0.02-0.50% S, >1.0-4.0% Cu, 0.1-4.0% Ni, 0.05-5.0% Cr, 0.0002-0.1% Al, 0.001-0.1% Ti, 0.0003-0.03% B, 0.0010-0.0200% N, and the balance Fe with inevitable impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001] The present invention relates to a non-heat treated steel to be used as hot forged, and more particularly to a non-heat treated steel having high strength and high toughness suitable for steel for machine structural use.

[0002]

2. Description of the Related Art Conventionally, in the manufacture of automobiles or mechanical structural parts requiring high strength and high toughness, the strength and toughness are formed after hot working with SCM435 or SCM440, which is an alloy steel for machine structural use. In general, a tempering treatment such as quenching and tempering is performed to impart the material. Since this heat treatment step requires a lot of time and energy, if this can be omitted, the cost can be greatly reduced,
It can respond to energy saving. Therefore, various proposals have been made to respond to this request.

[0003] For example, a ferrite-pearlite type non-heat treated steel obtained by adding about 0.10 mass% of V to a medium carbon Mn steel of 0.3 to 0.5 mass% is known. This non-heat treated steel precipitates V carbonitride in the cooling process after hot rolling, strengthens the ferrite ground, and utilizes the strength of pearlite to increase the strength of the entire steel.

Further, Japanese Patent Publication No. Hei 6-6302 or Japanese Unexamined Patent Publication No.
In each publication of No. 371547, 0.05 to 0.3 mass% of low carbon
A non-heat treated steel for hot forging of a bainite type or a martensite type to which n, Cr, V or the like is added is disclosed.

[0005]

However, the former non-heat treated ferrite-pearlite type steel has a tensile strength of 0.3 to 0.5 mass%, which is present as cerite in pearlite. And it is difficult to achieve both toughness. In addition, in order to obtain stable quality, a complicated operation such as controlling a cooling rate after forming a component is required.

[0006] On the other hand, the latter bainite type non-heat treated steel has been proposed to compensate for the above-mentioned lack of toughness.
Hot forging has low yield strength and requires aging after the court. Further, this kind of steel can secure sufficient toughness with small parts, but has insufficient toughness with large parts having a slow cooling rate. Therefore, it is necessary to control the cooling rate in the bainite transformation temperature range. Further, in the conventional bainite type non-heat treated steel, there is also a problem that the toughness of a portion that cannot be processed in hot forging is lower than that of a portion that has been processed.

Therefore, according to the present invention, sufficient tensile strength and yield strength, and high toughness can be obtained even in a portion hardly affected by working without controlling the cooling rate or performing aging treatment after hot forging. The purpose is to propose steel for hot forging.

[0008]

Means for Solving the Problems The inventors of the present invention have intensively studied a method for solving the above-mentioned problems, and found that it is extremely advantageous to adjust the bainite structure as schematically shown in FIG. With this knowledge, the present invention was completed. That is, as shown in FIG. 1, a block structure is positively generated in the bainite structure, the toughness is improved even in a part that cannot be processed, and the strength of the steel is increased by the precipitation of Cu. It has been found that when quenching properties are improved by adding Mn, Ni, Cr, B, etc., high strength and toughness can be obtained without performing tempering treatment after hot forging.

[0009] In particular, Cu not only enables a marked increase in strength when the cooling rate is low, but also effectively works to improve machinability when used in combination with S in an appropriate range, and provides high strength and excellent machinability. Mn and Cu among Mn, Cu, Cr and B are extremely effective in promoting the generation of a block structure in the bainite structure, and the addition of these does not sufficiently extend the processing. It has been newly found that high toughness can be obtained even in a part.

The gist of the present invention is as follows. (1) C: more than 0.05 mass% and less than 0.15 mass%, Si: 0.005
mass% or more and 2.0 mass% or less, Mn: more than 2.0 mass% and 5.0
mass% or less, S: 0.02 mass% or more and 0.50 mass% or less, Cu:
More than 1.0 mass%, less than 4.0 mass%, Ni: more than 0.1 mass%
4.0 mass% or less, Cr: 0.05 mass% or more and 5.0 mass% or less,
Al: 0.0002 mass% or more and 0.1 mass% or less, Ti: 0.001 mass
% To 0.1 mass%, B: 0.0003 mass% to 0.03 mass
% And N: not less than 0.0010 mass% and not more than 0.0200 mass%, and the balance consists of Fe and unavoidable impurities.

(2) In the above (1), V: 0.5 ma
A non-heat treated steel for hot forging, characterized in that it contains one or two kinds of ss% or less and W: 0.5 mass% or less.

(3) In the above (1) or (2),
Mo: 0.05 mass% or more and 3.0 mass% or less and Nb: 0.005 ma
A non-heat treated steel for hot forging, characterized by containing one or two kinds of ss% or more and 0.15 mass% or less.

(4) In the above (1), (2) or (3),
Further, Zr: 0.02 mass% or less, Mg: 0.02 mass% or less, Hf:
A non-heat treated steel for hot forging, characterized by containing one or more kinds of not more than 0.1 mass% and REM: not more than 0.02 mass%.

(5) In the above (1), (2), (3) or (4), P: 0.10 mass% or less, Pb: 0.30 mass% or less, Co: 0.1 mass% or less, Ca: 0.02 mass% % Or less, Te: 0.
A non-heat treated steel for hot forging, comprising one or more of 05 mass% or less, Se: 0.10 mass% or less, Sb: 0.05 mass% or less, and Bi: 0.30 mass% or less.

[0015]

Next, the reasons for limiting each component in the non-heat treated steel of the present invention will be described in detail. C: more than 0.05 mass% and less than 0.15 mass% C requires an amount of more than 0.05 mass% to secure the strength and to form a block structure in the bainite structure, but if it exceeds 0.15 mass%, the toughness decreases. 0 to damage.
The range was more than 05 mass% and less than 0.15 mass%.

Si: 0.005 mass% or more and 2.0 mass% or less Si is at least 0.005 ma to achieve deoxidation and solid solution strengthening.
Since ss% is required, on the other hand, if added excessively, the toughness is reduced.

Mn: more than 2.0 mass% and not more than 5.0 mass% Mn contains more than 2.0 mass% in order to improve the hardenability and to have a bainite structure including a block structure to ensure strength and toughness. is necessary. On the other hand, if the content exceeds 5.0 mass%, the machinability deteriorates.
The upper limit is mass%.

S: 0.02% by mass or more and 0.50% by mass or less S is an element which enhances the machinability, particularly when added in combination with Cu, and it is necessary to add 0.02% by mass or more in order to exert its effect. However, if added in excess, the cleanliness becomes poor and the toughness is reduced, so the upper limit is made 0.50 mass%.

Cu: more than 1.0% by mass and not more than 4.0% by mass Cu enhances the machinability by precipitation strengthening and addition of a complex with S, and promotes the formation of a block structure in the bainite structure to improve toughness. Add to improve. In order to exhibit this effect, a content of more than 1.0 mass% is necessary, while if it exceeds 4.0 mass%, the toughness is sharply reduced.
The upper limit is 0 mass%.

Here, the experimental results of investigating the effects of Cu and S on machinability will be described in detail. That is, a plurality of steel blooms of various components shown in Table 1 were manufactured by continuous casting, each bloom was formed into a 50 mmφ steel bar by hot rolling, heated to 1250 ° C., and then formed into 30 mmφ by hot forging. The temperature range of 800 to 400 ° C was cooled at a cooling rate of 0.001 to 80 ° C / s.

[0021]

[Table 1]

Of the steel materials thus obtained, 0.1 ° C./s
FIG. 2 shows the results of evaluating the machinability of the steel material cooled in the above, in comparison with the drill life of the SCM435QT product specified in JIS 4105, which is a conventional steel. This machinability was evaluated by the total drilling depth until the drill was broken in a drill cutting test.
The cutting conditions were the number of revolutions using a 4mmφ high speed drill.
The test was performed under the conditions of 1500 rpm, feed amount of 0.10 mm / rev, and drilling depth of 12 mm / piece.

Observing the shape of the chips formed at this time, ◎ indicates that fine and fine chips are generated, ○ indicates that divided chips are generated, and 場合 indicates that intermittent continuous chips are generated. And △, the case where long chips were continuously generated and hindered the work was evaluated as ×, and the chip disposability was evaluated. FIG. 3 shows the evaluation results.

FIG. 2 shows that the drill life of the conventional JIS steel (approximately
In order to extend the tool life to 600 mm, which is twice as long as 300 mm), it is necessary to satisfy the conditions of Cu ≧ 1.0 mass% and S> 0.010 mass%. For this purpose, Cu> 1.0 mass% and S> 0.020 mass%
It became clear that it was necessary to satisfy the conditions. These appropriate ranges were similar for all cooling rates.

Next, FIG. 4 shows the relationship between the cooling rate after rolling and the tensile strength. As shown in FIG. 4, the cooling rate after rolling was about 5 ° C./s.
In the following cases, it can be seen that Cu is finely precipitated during the cooling process and effectively acts to increase the strength. In general ripe forging, the cooling rate after forging is 5 ° C./s or less. Therefore, if the steel of the present invention is used, the cooling rate after forging is not controlled, that is, non-refined and high strength. Obviously, this can be achieved.

FIG. 5 shows the effect of the added amount of Cu on the strength increase at a cooling rate of 0.6 ° C./s.
From the figure, when the Cu addition amount exceeds 1.0 mass%, ΔTS (Cu
It can be seen that the difference from TS in the case of no addition rapidly increases. Further, when Cu ≧ 1.5 mass%, a greater strength increasing effect can be obtained.

In the conventional steel, in the cooling process after hot forging, when the cooling rate is slower than when each of the forged parts is air-cooled singly due to, for example, overlapping of the forged parts, the structure becomes Is softened and the strength is insufficient, so that cooling management is required, and there is a problem that the manufacturing cost increases. In this regard, as shown in FIG. 4, the steel to which Cu is added according to the present invention can be used even when the cooling rate is low.
The softening of the structure can be suppressed to a small level by the strengthening of precipitation of Cu, and stable strength can be obtained, so that it is not necessary to control the cooling rate.

Ni: 0.1 mass% to 4.0 mass% Ni is an element effective for improving strength and toughness. When Cu is added, it also has an effect of preventing Cu cracking during rolling. . To achieve these effects, 0.1
Although it is necessary to contain more than mass%, it is expensive, and its effect is saturated even if it is added excessively.

Cr: 0.05 to 5.0 mass% Cr improves the hardenability and is effective in reducing the change in strength and toughness with respect to the cooling rate after hot forging. It is also effective in promoting the formation of a block structure after hot forging. For this reason, a content exceeding 0.05 mass% is necessary, but if added excessively, the toughness is reduced, so that the content is set to 5.0 mass% or less.

Al: 0.0002 mass% or more and 0.1 mass% or less Al acts not only as a deoxidizing agent, but also forms AlN together with N to refine the structure and improve the toughness. For this purpose, the content of 0.0002 mass% or more is necessary. However, if the addition amount exceeds 0.1 mass%, alumina-based inclusions increase and the toughness is impaired, so the upper limit is 0.1 mass%.

Ti: not less than 0.001 mass% and not more than 0.1 mass% In addition to precipitation strengthening, Ti forms TiN together with N to contribute to the refinement of the structure, has the effect of improving toughness, and acts as a deoxidizing agent. Also works. For this purpose, it is necessary to add 0.01 mass% or more. However, if added excessively, coarse TiN precipitates when the cooling rate is low, and on the contrary, the toughness is reduced. Therefore, the upper limit is 0.1 mass%.

B: 0.0003 mass% or more and 0.03 mass% or less B is an element for improving hardenability, and is effective for suppressing a change in strength and toughness with respect to a cooling rate.
Further, it is an essential component because it is effective in promoting the generation of a block structure after hot forging. In order to exert its effect, 0.0003 mass% or more must be added. On the other hand, even if it is added excessively, its effect is saturated, so the upper limit is 0.03 mass%.

N: 0.0010 mass% or more and 0.0200 mass% or less N forms AlN and TiN together with Al or Ti and precipitates, and forms a structure as a pinning site for suppressing crystal grain growth during heating such as hot forging. It works to refine and improve toughness. However, if the content is less than 0.0010 mass%, the effect of the precipitation of AlN and TiN cannot be sufficiently obtained.
When added in excess of 00 mass%, not only does the effect become saturated, but solute N rather reduces the toughness of the steel, so
The range is 10 to 0.0200 mass%.

Further, in the steel of the present invention, the following components can be added for the purpose of improving hardenability, toughness or machinability, and the addition amounts of the respective components are as follows.

First, in order to improve the strength, V: 0.5
It is effective to add one or two of the following mass% or less and W: 0.5 mass% or less. V: 0.5 mass% or less V forms V (C, N) and is used for precipitation strengthening, and V (C, N) precipitated in the austenite region is used as a bainite forming nucleus to form a microstructure. Contributes to miniaturization and improvement of toughness. However, if the addition exceeds 0.5 mass%, the effect is saturated and problems such as continuous casting cracks are caused.

W: not more than 0.5 mass% In addition to the increase in strength due to solid solution strengthening, W reacts with C to precipitate WC and contributes to the increase in strength. 0.5 mass to cause severe toughness reduction
% Or less.

Next, in order to improve the hardenability and improve the strength, Mo: 0.05 to 3.0 mass% and Nb: 0.005 to 0.15
It is effective to add one or two mass%. Mo: 0.05 to 3.0 mass% Mo has the effect of increasing the strength at room temperature and high temperature when added at 0.05 mass% or more. However, if added excessively, it will increase the cost. Added.

Nb: 0.005 to 0.15 mass% Nb is added in an amount of 0.005 mass% or more for the purpose of improving hardenability, precipitation strengthening and toughness. 0.005 to inhibit
It is added in the range of 0.15 mass%.

In order to act as a deoxidizing component and to improve toughness by making crystal grains fine, Z
r: 0.1 mass% or less, Mg: 0.02 mass% or less, REM: 0.02 m
One or more of ass% or less and Hf: 0.1 mass% or less can be added.

Zr: 0.1 mass% or less Zr acts as a deoxidizing agent and is effective in increasing the strength and toughness by refining the crystal grains.
Even if the content exceeds 0.1 mass%, the effect is saturated, so the content is set to 0.1 mass% or less.

Mg: 0.02% by mass or less Mg acts as a deoxidizing agent and is effective in increasing strength and toughness by refining crystal grains.
Even if the content exceeds 02 mass%, the effect is saturated, so the content is set to 0.02 mass% or less.

Hf: 0.1 mass% or less Hf is effective in increasing the strength and toughness by refining the crystal grains. However, if the content exceeds 0.10 mass%, the effect is saturated. % Or less.

REM: 0.02% by mass or less REM is effective in increasing the strength and toughness by refining the crystal grains.
Since the effect is saturated, the content is set to 0.02 mass% or less.

Further, in order to improve the machinability, P:
0.10 mass% or less, Pb: 0.30 mass% or less, Ca: 0.02 mass%
Te: 0.05 mass% or less, Co: 0.10 mass% or less, Se:
0.10 mass% or less, Sb: 0.05 mass% or less and Bi: 0.30 ma
One or more of ss% or less can be contained.

P: 0.10 mass% or less P is added for the purpose of improving the machinability, but excessive addition adversely affects toughness or fatigue resistance.
mass% or less, preferably 0.07 mass% or less.

Pb: 0.30% by mass or less Pb is an element which has a low melting point and exerts a liquid lubricating effect when melted by heat generation of a steel material at the time of cutting to improve the machinability. Since the effect is saturated and the fatigue resistance is reduced, the content is set to 0.30 mass% or less.

Ca: 0.02 mass% or less Ca is an element having almost the same effect as Pb.
When the mass% is exceeded, the effect is saturated, so 0.02mass
% Or less.

Co: 0.10 mass% or less Co is a component having substantially the same effect as Pb and Ca.
If it exceeds 0.10mass%, the effect will be saturated.
mass% or less.

Te: 0.05 mass% or less Te is a component that improves machinability like Pb and Ca.
If the content exceeds 05 mass%, the effect is saturated and the fatigue resistance is reduced. Therefore, the content is set to 0.05 mass% or less.

Sb: 0.05 mass% or less Sb is a component having substantially the same effect as Co, Pb, Ca and Te. However, if the content exceeds 0.05 mass%, the effect is saturated, so that the content becomes 0.05 mass% or less. I do.

Bi: 0.30% by mass or less Bi is a component having substantially the same effect as Sb, Co, Pb, Ca and Te. However, if it exceeds 0.30% by mass, the effect is saturated, so that 0.03% by mass is obtained. The following is assumed.

Se: 0.10 mass% or less Se combines with Mn to form MnSe, which acts as a chip breaker to improve machinability. However, if the addition exceeds 0.10 mass%, the fatigue resistance is adversely affected, so the content is set to 0.10 mass% or less.

In the above-mentioned added components, the lower limit is not particularly defined for the component which exerts its effect even in a trace amount.

The non-heat-treated steel of the present invention can provide not only high strength and high toughness but also excellent machinability, even when the cooling rate after hot forging is low, by adjusting the components to the above basic composition. Therefore, it is not necessary to strictly control the cooling rate after hot forging, and it is sufficient to manufacture according to general mechanical structural parts.

For example, after a hot-rolled steel bar whose composition is adjusted to the above-described basic composition is heated to 1250 ° C., a predetermined shape is obtained by hot forging in a temperature range of 1050 to 1250 ° C., and then cooled or gradually cooled. As a result, desired characteristics can be obtained.

No special treatment is required after this hot forging, but after cooling to room temperature after hot forging,
By performing the reheating treatment for 30 s or more in a temperature range of less than 800 ° C., the strength can be increased.

[0057]

EXAMPLES Steels having various component compositions shown in Tables 2 to 4 were melted in a converter, converted into blooms by continuous casting, and then hot-rolled into steel bars of 65 mmφ. Then, after heating to 1250 ° C,
Formed to 30mmφ by hot forging, and temperature range of 800-500 ° C
The cooling was performed at a cooling rate of (1) 0.05 ° C / s, (2) 0.5 ° C / s, and (3) 5 ° C / s. In addition, assuming a part that cannot be processed in hot forging, a steel bar of 65 mmφ is cut by 30 mm
After processing into a φ steel bar and heating it to 1250 ° C, the cooling rate
Cooling at 30 ° C / min was also performed. Further, some of these steel bars were heat-treated at 550 ° C for 40 minutes. From the bar steel thus obtained, a tensile test piece (JIS4
No.) and impact test pieces (JIS No. 3) were sampled and mechanical properties were investigated. Tables 5 to 10 show the results of the investigation. The machinability was evaluated in the same manner as in the experiment shown in FIG. 2, and the chip disposal was evaluated in the same manner as in the experiment shown in FIG.

[0058]

[Table 2]

[0059]

[Table 3]

[0060]

[Table 4]

[0061]

[Table 5]

[0062]

[Table 6]

[0063]

[Table 7]

[0064]

[Table 8]

[0065]

[Table 9]

[0066]

[Table 10]

As shown in Tables 5 to 10, the steel according to the present invention has a high tensile strength TS ≧ 923 MPa at any cooling rate and in cases where the steel cannot be worked, and even when the strength becomes high. toughness is extremely good and _u E ₂₀ ≧ 92J / cm ² or more, exhibited excellent drill life than even the steel 50 and 51 is a conventional non-heat treated steel for machinability.

On the other hand, in the steel 34 as a comparative example, since the C content did not reach the lower limit of the present invention, the toughness in the case where the working was not performed was lowered. On the other hand, Steel 35 had a lower toughness because the C content was higher than the upper limit of the present invention. Steel 36 had a lower toughness because the Si content was higher than the upper limit of the present invention. steel
In No. 37, since the Mn content did not reach the lower limit of the present invention, the strength and the toughness in the case where the working was insufficient were reduced.

Similarly, since the steel 38 had a higher Mn content than the upper limit of the present invention, the machinability was lowered. Steel 39 had hot brittleness during rolling because the Ni content did not reach the lower limit of the present invention. In Steel 40, since the Cu content did not reach the lower limit of the present invention, the strength, toughness, machinability, and chip disposition decreased. Steel 41 has a Cu content higher than the upper limit of the present invention,
The toughness decreased. In steel 42, since the S content did not reach the lower limit of the present invention, the machinability and the chip disposability decreased. steel
In No. 43, since the Cr content was higher than the upper limit of the present invention, the toughness was lowered. Steel 44 had a lower toughness because the Ti content was higher than the upper limit of the present invention. In steel 45, since the B content did not reach the lower limit of the present invention, the formation of a block structure was not sufficient and the toughness was lowered. Steel 46 had a lower toughness because the N content was higher than the upper limit of the present invention.

The strength and toughness of steel 48, which is a conventional non-heat treated steel, largely depends on the cooling rate. That is, the steel 48 having a ferrite-pearlite structure has a low TS of 766 MPa even when the cooling rate is high, and further lowers when the cooling rate is low. The toughness is about 40 J / cm ² even when the cooling rate is high, and 30 toughness when the cooling rate is low.
It stays at around J / cm ² .

In this respect, even with the conventional non-heat treated steel 47, although the balance between strength and toughness is better than the steel 48 at any cooling rate, the conventional heat treated steel 49, Compared to the 50 and invention steels, each of them is at a lower level.
In other words, it can be seen that the steels 48 and 47 may be applicable to small parts having a relatively high cooling rate, but are not suitable for large parts having a low cooling rate. Further, the comparative steel 47 has a lower toughness when no processing is applied than the tempered steel or the inventive steel. On the other hand, the mechanical properties and toughness of the invention steel have a very low cooling rate dependency, and when the part shape changes, for example, when it becomes a large cross-sectional shape or even in a part where processing is not sufficiently performed. , Sufficient strength and toughness can be imparted evenly.

[0072]

The non-heat treated steel of the present invention has a better balance of strength and toughness than conventional non-heat treated steel.
It can be widely used for various mechanical parts such as important safety parts or shafts, rolling parts and sliding parts for automobiles that require high strength and high toughness.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a bainite structure of a non-heat treated steel of the present invention.

FIG. 2 is a diagram showing a relationship between Cu and S contents and drill life.

FIG. 3 is a diagram showing a relationship between Cu and S contents and chip disposability.

FIG. 4 is a diagram showing the effect of the cooling rate after rolling on the tensile strength of a rolled material.

FIG. 5 is a diagram showing the influence of Cu on the increase in strength.

Continuing from the front page (72) Inventor Kenichi Amano 1-chome, Kawasaki-dori, Mizushima, Kurashiki-shi, Okayama Pref.

Claims (5)

[Claims] C: more than 0.05 mass% and less than 0.15 mass%, Si: 0.005 mass% to 2.0 mass%, Mn: more than 2.0 mass% to 5.0 mass%, S: 0.02 mass% to 0.50 mass% , Cu: 1.0 mass% to 4.0 mass% or less, Ni: 0.1 mass% to 4.0 mass%, Cr: 0.05 mass% to 5.0 mass%, Al: 0.0002 mass% to 0.1 mass%, Ti: 0.001 mass % To 0.1 mass%, B: 0.0003 mass% to 0.03 mass% and N: 0.0010
A non-heat treated steel for hot forging, characterized by containing not less than mass% and not more than 0.0200mass%, the balance being Fe and unavoidable impurities. 2. The method according to claim 1, further comprising: V: 0.5 mass
% Or less and W: 0.5 mass% or less. 3. The method according to claim 1, further comprising:
0.05 mass% or more and 3.0 mass% or less and Nb: 0.005 mass%
A non-heat treated steel for hot forging, characterized in that it contains at least one kind of at least 0.15 mass%. 4. The method according to claim 1, 2 or 3, further comprising:
A non-heat-treated steel for hot forging, characterized by containing one or more of Zr: 0.02 mass% or less, Mg: 0.02 mass% or less, Hf: 0.1 mass% or less, and REM: 0.02 mass% or less. 5. The method according to claim 1, wherein P: 0.10 mass% or less, Pb: 0.30 mass% or less, Co: 0.1 mass% or less, Ca: 0.02 mass% or less, Te: 0.05 mass%. A non-heat treated steel for hot forging, characterized by containing one or more of Se: 0.10 mass% or less, Sb: 0.05 mass% or less, and Bi: 0.30 mass% or less.